1. Field of the Invention
The present invention relates to a reference voltage generating circuit configured to generate a predetermined reference voltage and a reference voltage source including the reference voltage generating circuit, particularly to a reference voltage generating circuit and a reference voltage source each having an excellent temperature characteristic.
2. Description of the Related Art
Reference voltage generating circuits configured to stably supply a predetermined reference voltage at any temperature have been known. FIG. 14 is a circuit diagram showing a basic configuration of a conventional reference voltage generating circuit. As shown in FIG. 14, a reference voltage generating circuit 110 includes a first path P10, a second path P20, and a differential amplifier 40. On the first path P10, a first diode characteristic element Q10, such as a diode or a bipolar transistor, and a first resistor R10 are connected in series to each other. The first diode characteristic element Q10 has a diode characteristic (current-voltage characteristic by PN junction). On the second path P20, a second diode characteristic element Q20 and a second resistor R20 are connected in series to each other. The density of a current flowing through the second diode characteristic element Q20 is different from that of a current flowing through the first diode characteristic element Q10. To the differential amplifier 40, a voltage V10 obtained after the voltage drop by the first resistor R10 and a voltage V20 obtained after the voltage drop by the second resistor R20 are input. Further, on the second path P20, a third resistor R30 is connected in series to the second resistor R20. Then, a voltage (in the example shown in FIG. 14, an output voltage of the differential amplifier 40) applied to the first resistor R10 and the second resistor R20 is output as a reference voltage VBG. In the reference voltage generating circuit configured as above, the third resistor R30 (and the second resistor R20) is adjusted based on a difference between voltages, respectively applied to the diode characteristic elements Q10 and Q20 which are different in the density of the flowing current from each other, such that temperature dependence of the reference voltage VBG is eliminated (such that a differential dVBG/dT of the reference voltage VBG by a temperature T becomes zero).
It is known that a fluctuation range of the obtained reference voltage VBG by temperatures becomes narrow, but strictly, the obtained reference voltage VBG quadratically fluctuates by temperatures. FIG. 15 is a graph showing a temperature dependence characteristic of the reference voltage obtained by the conventional reference voltage generating circuit. FIG. 15 shows that the reference voltage has a quadratic temperature dependence characteristic in an assumed temperature range (−50 to 150° C.). This is because although a first-order temperature coefficient of the reference voltage is canceled by the reference voltage generating circuit shown in FIG. 14, a second-order temperature coefficient of the reference voltage still exists.
As a method of eliminating this quadratic temperature dependence characteristic, it has been thought in theory that a current which quadratically fluctuates by temperatures is caused to flow through a current path of the reference voltage generating circuit shown in FIG. 14. However, if the circuit is configured to generate the current which quadratically fluctuates in accordance with the quadratic temperature dependence characteristic, it becomes complex, and such circuit is not realistic.
Here, to eliminate the temperature dependence characteristic, for example, a configuration has been proposed, in which a plurality of correction current generating circuits are provided, and correction currents respectively generated by the correction current generating circuits are respectively used in a plurality of temperature ranges (see PTL 1 for example). In addition, another configuration has been proposed, in which a PTAT current which linearly changes with respect to an absolute temperature is generated, and temperature compensation is performed by performing adjustments such that a difference between the PTAT current and a CTAT current proportional to a voltage applied to the diode characteristic element by using the PTAT current and a resistor becomes zero (see PTL 2 for example).